(Edit - I see things have moved on since I started this reply. Upstream sync has picked up but downstream fallen. Leave it alone for now

).
Very odd, and annoying for you.
As MrSaffron says, there is no direct DLM control by Sky on FTTC, but it is possible they are running some sort of IP Profile system of their own. At one point, maybe even these days, people going on ADSLx with them would be started at exactly 4Mbps then ramped up in two or three steps over the next few days. (On their LLU ADSLx they do have direct DLM control as it is completely their system. On FTTC the only DLM is Openreach's).
Those speed tests do look like a solid cap but lower than I would expect even for 40Mbps. Keep an eye on the sync speeds - have you checked it hasn't reverted to 40000?
I'm not sure whether you should do the re-sync I suggested, or perhaps wait the ten days, or a power cycle of the router as has also been suggested. Maybe do that in broad daylight on Saturday, leaving the router turned off for maybe 30 minutes when you do.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 54999/14466Kbps @ 600m. BQMs -
IPv4 &
IPv6
Edited by RobertoS (Wed 15-Feb-17 10:32:16)